Remove the cartridge caddy by removing the 6 screws that hold it in place. Take note that the 2 silver screws are longer and remember to put them back in the same spot.
Nintendo NES with top cover removedNintendo NES view of game cartridge caddyNES Repair remove game cartridge caddy step: aNintendo NES remove game cartridge caddy step: bNintendo NES View of Motherboard and 72 pin connector. Game cartridge caddy removed.

Remove the last 2 screws that are located near where the power supply and component cables plug in.

Now you can remove the motherboard from the NES.
Nintendo NES Motherboard with 72 pin connector removedNintendo NES Motherboard Removed